In this study we will test the hypothesis that polydipsia, per se, does not alter vasopressin secretion &/or action. A standard oral water load & hyper- tonic saline infusion will be given to polydipsic schizophrenics with & without hyponatremia, plus 2 other control groups. If the hypothesis is correct only the polydipsic hyponatremic patients will exhibit our previously defined changes in osmoregulation of vasopressin & impaired urinary dilution.